It has previously been proposed to improve the security of a vehicle by incorporating in an ignition key an electronically readable code which verifies that the key is authentic, and preventing vehicle operation when the correct code is absent. Thus a vehicle theft cannot be accomplished by merely copying the key shape.
The code device incorporated in the key is a miniature transponder which is well known as an implant in animals used for identification of individual animals. The implant comprises windings on a ferrite core to serve as an antenna, and an electronic circuit for transmitting serial data comprising the identification code. An external coil positioned to inductively couple with the antenna irradiates an AC signal of typically 125 kHz to energize the transponder, providing sufficient power for its operation. The serial data signal emitted by the transponder is received by the external coil and then analyzed by an associated electronic circuit to determine the code.
The usage of this technology for vehicle ignition keys has been carried out by a wire coil wound on a bobbin which surrounds the ignition lock cylinder, and the coil terminations are connected to an electronic circuit for supplying the AC power to the coil to excite the transponder and for reading the code emitted by the transponder. This implementation of the reader/exciter coil leads to two problems: first, the shape and bulk of the coil and bobbin is not always compatible with the space available adjacent the lock cylinder; and second, the connection of the coil terminations to the electronic circuit is a source of electrical faults.